1885.] "^ (Genth. 



The atomic ratio leads to the accepted formula : 



4(Ag 2 Cu 2 ZnFe) S. (SbAsBi) 2 S 3 . 



An exceedingly interesting association of this tetrahedrite is that of 

 sylvanite. There was in the lot of samples from Governor Pitkins' Mine 

 a small piece of quartz, showing very few specks of tetrahedrite and also 

 the antimony oxide coating, which showed a few silver-white, bright me- 

 tallic particles, 2-3 mm in length and less than l mm broad, with one eminent 

 cleavage. A qualitative examination showed that, when nitric acid was 

 added, the particles at once became black, and on boiling dissolved, leav- 

 ing bright brown gold ; the filtrate gave an abundance of silver chloride 

 on addition of hydrochloric acid, and the filtrate evaporated to dryness 

 yielded crystalline tellurous oxide, readily soluble in ammonium hy- 

 drate and ammonium sulphide, which latter solution, on acidulation, gave 

 a black precipitate of tellurous sulphide. 



Thus the mineral is proved to be sylvanite, which, I do not believe, has 

 ever before been observed in this part of Colorado. 



100.00 



9. Arsenopyrite and Scorodite. 



A number of years ago I received, from Northern Alabama, several 

 lumps of ore, consisting of quartz with pale grayish-green granular scoro- 

 dite, showing on some of the fissures microscopic crystals of the usual 

 form of this mineral, the pyramidal planes predominating. The scorodite 

 results from the oxidation of arsenopyrite, a portion of which is left un- 

 altered in the form of grayish-white, granular patches with metallic lus- 

 tre. Dilute hydrochloric acid dissolved the scorodite and left the arseno- 

 pyrite with a little quartz. After deducting 3.34 per cent of the latter the 

 analysis gave : 



Sulphur = 



Arsenic = 



Iron = 



Copper = 



1:1:1 = FeSAs. 



